Luminophore plate

ABSTRACT

A luminophore plate has a substrate and an auxiliary layer lying thereabove onto which a storage luminophore layer is applied. The auxiliary layer is rastered such that nubs separated by trenches are formed. Luminophore needles of a storage luminophore are formed on the surface of said nubs by vapor deposition.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to a luminophore plate of thetype having a substrate and an auxiliary layer lying thereabove ontowhich a storage luminophore layer is applied.

[0003] 2. Description of the Prior Art

[0004] X-ray luminophores are generally employed in medical technologyand in non-destructive materials inspection. In these applications,scintillators are utilized for spontaneous emission under X-raystimulation, and storage luminophores are utilized for forming andstoring electrons and holes for subsequent photo-stimulated emission(PSL) when irradiated with, for example, red light.

[0005] X-ray luminophores on the basis of alkali halides play a veryspecific role for such purposes. Examples are CSI:Na in X-ray imageintensifier, Csl:TI in a-Si detectors and, recently CsBr:Eu as a storageluminophore plate as described, for example, in Proc. SPIE, Vol. 4320(2001), “New Needle-crystalline CR Detector” by Paul J. R. Leblans etal., pages 59-67.

[0006] All of the aforementioned medical applications of alkali halideshave in common the fact that the layers are produced by thermalevaporation of the alkali halides (CsBr, Csl) and of the respectivedopants (Tll, Nal, EuBr2). Dependent on the vapor pressure of thematerials, the substances can be evaporated from one or from twoevaporator vessels, as described, for example, in German OS 100 61 743and German PS 195 16 450.

[0007] To achieve a needle-shaped layer structure which has the abilityto guide light, the vapor depositions disclosed in the above prior artpublications usually is implemented at an elevated substratetemperature. The coefficient of thermal expansion of the alkali halidesCsl and CsBr that are utilized lies at 4.8×10⁻⁵/° C. Glass, steel,nickel, titanium, copper and aluminum oxide ceramic can be utilized assubstrates, their coefficients of thermal expansion be set forth in thefollowing Table from D'Ans Lax, Taschenbuch f{overscore (u)}r Chemikerund Physiker, Volume 1. TABLE 1 Substrate material Coefficient ofthermal expansion Glass (0.3-0.9) × 10⁻⁵/° C. Aluminum 2.4 × 10⁻⁵/° C.Steel (1.0-1.8) × 10⁻⁵/° C. Nickel 1.3 × 10⁻⁵/° C. Titanium 0.8 × 10⁻⁵/°C. Copper 1.7 × 10⁻⁵/° C. Aluminum oxide 0.8 × 10⁻⁵/° C.

[0008] Shrinkage cracks arise in the luminophore layers when cooling thevapor-deposited substrates due to the lower coefficients of thermalexpansion. The frequency of crack occurrence is on the order ofmagnitude Of 0.5-1.5 mm, as can be seen from FIG. 1 herein that shows a50-power scanning electron microscope image of a known CsBr:Eu layer.The cracks have a width of up to approximately 10 μm, as can be seen inFIG. 2 that shows a 1000-power scanning electron microscope image of aknown CsBr:Eu layer.

[0009] As is known, more light is coupled out of a luminophore layer atgrain boundaries, gaps and cracks than from the luminophore needlesthemselves. The corona exposure according to FIG. 3 shows a microscopicillustration of an incident light point of a CsBr:Eu layer withnoticeably brighter gaps and a crack that demonstrates this behavior.

[0010] The problem area that has been described is especially pronouncedfor storage luminophore layers such as, for example, CsBr:Eu. In thereadout event, the surface of the luminophore layer is thereby scannedwith a “red light spot” having a diameter of 50-150 μm. In the case of aglass substrate, however, the scanning also can ensue from the“underside”. A non-uniform readout of the stored electron-hole pairsthereby occurs corresponding to the layer structure. FIG. 4 shows thefrequency-dependent quantum efficiency (DQE) of a storage luminophorelayer. The “unnatural” course of the DQE curve from high to low spatialfrequencies can be clearly recognized. A “forced” plateau is present inthe region around 1 LP/mm. This effect is even noticeably intensifiedgiven a higher X-ray dose.

[0011] In contrast thereto, the light in the luminophore needles isgenerated by the X-ray quanta in the luminophore layers such as, forexample, Csl:Na or Csl:TI. The effect of a layer structure is not asproblematical, especially when no photocathode is present on theluminophore needles—as in the case of an X-ray image intensifier.

[0012] In context of the aforementioned prior art, attempt has been madeto generate many fine gaps around each luminophore needle by admittinggas during the vapor deposition process of the luminophore layers. Ithas been shown in practice, however, that this hope is achieved onlyconditionally, as a cathode luminescence exposure of a known CsBr:Eulayer manufactured according to German OS 100 61 743 magnified 250times, shown in FIG. 5, demonstrates.

[0013] Another disadvantage of this evaporation method is that thedensity of the layer becomes lower the higher the gas pressure is duringthe vapor deposition. This results in the geometrical layer thicknessincreasing by approximately 20% for identical X-ray absorption andincreased “crosstalk” of light in neighboring regions thus becomespossible. The MTF degradation is exactly as high as given a 20% thickerlayer with “normal” density and correspondingly higher X-ray absorptionand DQE.

[0014] German OS 29 29 745 discloses the manufacture of a luminescentscreen with a grid structure. An attempt was made to prescribe theneedle size with the grid structure of the substrate material by meansof a designational roughening. Each “nub”—original surface of thesubstrate material—is followed by a “trench”, an etched-in depression.This means that mammography applications are practically not possiblebecause of the small structural size that is required. Moreover, thestructuring method is highly material-dependent because of differentetching solutions, which is unfavorable in terms of fabricationtechnology and not environmentally compatible.

SUMMARY OF THE INVENTION

[0015] An object of the present invention is to eliminate theaforementioned disadvantages of the grid method, so that a mammographyapplication is possible and no material dependency exists.

[0016] This object is inventively achieved in a luminophore plate havinga substrate covered by an auxiliary layer, that is in turn covered by astorage luminophore layer, wherein the auxiliary layer is rastered suchthat nubs separated by trenches are formed, and luminophore needles ofthe storage luminophore are formed on the surface of the nubs by meansof vapor deposition. The material-dependency is avoided by rastering notthe substrate but an auxiliary layer lying thereabove.

[0017] One luminophore needle or a number of separate luminophoreneedles can be formed on each nub.

[0018] It has proven advantageous fir the auxiliary layer to be between20 and 200 μm thick. It can be inventively composed of a material whosecoefficient of thermal expansion lies between 2.5×10⁻⁵/° C. and4.7×10⁻⁵/° C.

[0019] Advantageously, the grid dimension (nub with trench) can lie inthe range between 10 and 100 μm, preferably 20—50 μm, with the width ofthe trenches being in the range from 5 to 20 μm.

[0020] It has been found to be especially suitable to compose theauxiliary layer of a plastic, for example of polyimide with acoefficient of thermal expansion of 3.1-3.5×10⁻⁵/° C. or parylene C.

[0021] Inventively, the nubs can be arranged in a grid structure thatvaries over the overall surface.

[0022] The nubs and/or their grid structure can be fashioned as n-sidedpolygons, whereby n can assume a number between three and six.

DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1, as described above, is a 50-power scanning electronmicroscope image of a known CsBr:Eu layer for illustrating the frequencyof crack occurrence.

[0024]FIG. 2, as described above, is a 1000-power scanning electronmicroscope image of a known CsBr″Eu layer for illustrating the crackwidth.

[0025]FIG. 3 is a microscope image of a light well of a known CsBr:Eulayer.

[0026]FIG. 4 is a curve of the frequency-dependent DQE of a CsBr:Eulayer with “plateau” through the known structure.

[0027]FIG. 5 is a 250-power cathode luminescence exposure of a knownCsBr:Eu layer.

[0028]FIG. 6 is a cross-section through a first embodiment of aninventive luminophore plate.

[0029]FIG. 7 is a cross-section through a second embodiment of aninventive luminophore plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030]FIG. 6 shows an inventive luminophore plate, for example a storageluminophore plate, that has a substrate 1 of glass or aluminum. Inaccordance with the invention an auxiliary layer 3 is vapor-depositedonto the substrate 1, and a storage luminophore layer 3 isvapor-deposited on said auxiliary layer 2, and the auxiliary layer 2 israstered such that nubs 5 separated by trenches 4 and formed. Due to thevapor-deposition of the storage luminophore onto the substratestructured by this auxiliary layer 2, individual, needle-shaped crystalsof the storage luminophore, referred to as luminophore needles 6,respectively form on the nubs 5, with the nubs functioning as “seeds,”the said needles 6 being separated by interstices 7.

[0031]FIG. 7 shows a further embodiment of the inventive storageluminophore plate that has essentially the same structure as theembodiment of FIG. 6 but exhibits a larger intermediate layer grid giventhe same needle size. A number of separate luminophore needles 6 areformed here on each of the nubs 5 by selecting the coefficients ofthermal expansion and the vapor-deposition conditions.

[0032] The auxiliary layer 2 must be between 20 and 100 μm thick andshould be composed of a material with a coefficient of thermal expansionbetween 2.5×10⁻⁵/° C. and 4.7×10⁻⁵/° C. Advantageously, the auxiliarylayer 2 is composed of a plastic, particularly parylene C or a polyimidelayer having a coefficient of thermal expansion of 3.1-3.5×10⁻⁵/° C. Thestructuring with various patterns, for example quadratic or hexagonal,can be implemented with the currently available methods, for examplephotolithography/etching, electron beam, laser beam or ion beamvaporization. However, the nubs 5 and/or their structure can alsoexhibit other shapes. They can also be triangular, pentagonal orpolygonal. The given structure can also vary over the surface.

[0033] The grid dimension generated in this way, i.e. the distance fromone nub 5 with a trench 4 to the next, should lie between 10 and 100 μm,particularly 20-50 μm. The width of the trench 4 should amount tobetween 5 and 20 μm.

[0034] The selection of a suitable auxiliary layer 2—because of theexpansion—results in the CsBr layer shrinking more relative to the“structure layer’ 2 and the “structure layer” 2 shrinks more greatlyrelative to the substrate 1 when cooling the storage luminophore layer 3after the vapor deposition. For the first described cooling process,this results in that the luminophore needles 6 can be separated from oneanother when a number of the luminophore needles 6 are located on thestructured auxiliary layer 2. In the second described case, a gap orinterstice 7 can form around each nub 5. The shrinking process istwo-stage; the formation of non-uniformly coarse cracks, as is the casein the plate and manufacturing method disclosed in German OS 29 29 745,fails to occur.

[0035] A fine structure within the nubs 5 that is suitable formammography can be achieved when the vapor deposition is implemented atelevated gas pressure, as disclosed in German OS 100 61 743. Theelevated gas pressure can be achieved either by means of gas admissionor by employing a pump system with a correspondingly high finalpressure. It has been shown in the experiments that it is beneficial toimplement the vapor deposition at a gas pressure markedly below 1 Pa—contrary to the details in German OS 100 61 743. Moreover, theundesired “artificial” increase in layer thickness can be alleviated andno cracks in the CsBr layer can then be detected transversely across thenubs of the substrate. The cracks only occur when the layer morphologyis too “loose”, i.e. the vapor deposition was implemented at high gaspressure (>1 Pa).

[0036] A colorant solution can be uniformly introduced into theinterstices 7 over the entire layer surface in the finely structuredstorage luminophore layer 3 that has now been obtained. The colorantshould have the complementary color of the light wavelength that definesthe resolution. Blue for the red stimulation light in the case of theCsBr:Eu storage luminophore and red for the blue emission light in thecase of the Csl:Na scintillator. The solvent for the colorant must notdissolve the alkali halide that is employed. This method of introducinga colorant into the interstices 7 of the storage luminophore isdisclosed in German PS 44 33 132 but could not be implemented inpractice because a homogeneous drive-in of the colorant solution was notpossible due to the cracks.

[0037] By selecting a suitably structured auxiliary layer 2 on asubstrate 1, a luminophore layer or storage luminophore layer 3 can bestructured as desired coarsely as well as finely under suitableparameters (pressure, substrate temperature) during cooling after thevapor deposition.

[0038] Only a few exemplary embodiments for manufacturing the inventiveluminophore plate are described below among all of the possiblecombinations of substrate, auxiliary layer and the vapor deposition ofthe luminophore layer:

[0039] a) CsBr:Eu—vapor deposition at 0.5 Pa and a substrate temperatureof 150° C. on an aluminum substrate structured with parylene C (40 μmnubs with 10 μm trenches).

[0040] CsBr layer thickness 500 μm.

[0041] b) CsBr:Eu—vapor deposition at 0.ooo5 Pa and a substratetemperature of 300° C. on a glass substrate structured with parylene C(10 μm nubs with 10 μm trenches).

[0042] CsBr layer thickness 100 μm

[0043] c) CsBr:Eu—vapor deposition at 0.9 Pa and a substrate temperatureof 260° C. on an aluminum substrate structured by parylene C (30 μm nubswith 5 μm trenches).

[0044] CsBr layer thickness 300μ

[0045] d) CsBr:Eu—vapor deposition at 0.1 Pa and a substrate temperatureof 250° C. onto an aluminum substrate structured with polyimide PyralinPI 2611 (40 μm nubs with 10 μm trenches)

[0046] CsBr layer thickness 500 μm

[0047] e) CsBr:Eu—vapor deposition at 0.01 Pa and a substratetemperature of 180° C. onto a glass substrate structured with polyimidePyralin PI 2611 35 μm nubs with 10 μm trenches.

[0048] CsBr layer thickness 100 μm

[0049] f) CsBr:Eu—vapor deposition at 0.8 Pa and a substrate temperatureof 220° C. onto an aluminum substrate structured with polyimide PyralinPI 2611 (25 μm nubs with 5 μm trenches.

[0050] Csl:TI or Csl:Na can be utilized instead of CsBr:Eu. Thematerials listed in Table 1 can be utilized as substrate materials. Thepressure can lie between 0.0005 and 0.9 Pa. The substrate temperatureshould amount to between 150 and 300° C.

[0051] The CsBr:Eu layers can be subsequently colored with a solution,for example an ethanol or isopropanol solution of a “GEHA felt pen No.204, blue”. A solution designated “Marker edding 300, col 002, red” issuitable for the CsBr:Na layers. An anhydrous state must be achieved anda drying agent should be utilized as needed.

[0052] The invention results in a luminophore plate with a substrate andan auxiliary layer lying thereabove onto which the storage luminophorelayer is applied, wherein disturbing cracks in the luminophore layersare reduced by means of substrate structuring for increasing the quantumefficiency (DQE), particularly of storage luminophore layers.

[0053] Although modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

We claim as our invention:
 1. A luminophore plate comprising: asubstrate; an auxiliary layer disposed on said substrate, said auxiliarylayer being rastered to form a plurality of alternating nubs andtrenches; and a storage luminophore layer applied on said auxiliarylayer, said storage luminophore layer comprising luminophore needles ofa storage luminophore formed on the respective nubs of said auxiliarylayer by vapor deposition.
 2. A luminophore plate as claimed in claim 1wherein each of said nubs has a plurality of said luminophore needlesformed thereon.
 3. A luminophore plate as claimed in claim 1 whereinsaid auxiliary layer has a thickness in a range between 20 and 100 μm.4. A luminophore plate as claimed in claim 1 wherein said auxiliarylayer is composed of a material having a coefficient of thermalexpansion in a range between 2.5×10⁻⁵/° C. and 4.7×10⁻⁵/° C.
 5. Aluminophore plate as claimed in claim 1 wherein said auxiliary layer israstered with a grid dimension defined by said nubs and trenches in arange between 10 and 100 μm.
 6. A luminophore plate as claimed in claim5 wherein each of said trenches has a width in range between 2 and 20μm.
 7. A luminophore plate as claimed in claim 1 wherein said auxiliarylayer is composed of a plastic.
 8. A luminophore plate as claimed inclaim 1 wherein said auxiliary layer is composed of polyimide having acoefficient of thermal expansion in a range between 3.1×10⁻⁵/° C. and3.5×10⁻⁵/° C.
 9. A luminophore plate as claimed in claim 1 wherein saidauxiliary layer is composed of parylene C.
 10. A luminophore plate asclaimed in claim 1 wherein said auxiliary layer is rastered with a gridstructure formed by said nubs and trenches that varies over a surface ofsaid auxiliary layer onto which said storage luminophore layer isapplied.
 11. A luminophore plate as claimed in claim 1 wherein each ofsaid nubs has a shape of an n-sided polygon.
 12. A luminophore plate asclaimed in claim 11 wherein n is between 3 and
 6. 14. A luminophoreplate as claimed in claim 1 wherein said auxiliary layer is rasteredwith a grid structure of said nubs and trenches formed by a plurality ofn-sided polygons.
 15. A luminophore plate as claimed in claim 14 whereinn is between 3 and
 6. 16. A luminophore plate as claimed in claim 1wherein each of said nubs has a shape of an n-sided polygon and whereinsaid auxiliary layer is rastered in a grid structure of said nubs andtrenches formed by a plurality of n-sided polygons.
 17. A luminophoreplate as claimed in claim 16 wherein n is between 3 and
 6. 18. A methodfor manufacturing a luminophore plate comprising the steps of: disposingan auxiliary layer on a substrate, said auxiliary layer having an uppersurface facing away from said substrate; rastering said upper surface ofsaid auxiliary layer by forming a plurality of alternating nubs andtrenches at said upper surface of said auxiliary layer; and applying astorage luminophore layer onto said upper surface of said auxiliarylayer by vapor depositing luminophore needles of a storage luminophoreon each of said nubs.
 19. A method as claimed in claim 18 comprisingvapor depositing a plurality of said luminophore needles on each of saidnubs.
 20. A method as claimed in claim 18 comprising rastering saidupper surface of said auxiliary layer with a grid dimension of said nubsand trenches in a range between 10 and 100 μm.
 21. A method as claimedin claim 20 comprising rastering said upper surface of said auxiliarylayer with said grid dimension in a range between 20 and 50 μm.
 22. Amethod as claimed in claim 20 comprising forming each of said trencheswith a width in a range between 2 and 20 μm.
 23. A method as claimed inclaim 18 comprising rastering said auxiliary layer with grid structurethat varies over said upper surface of said auxiliary layer.
 24. Amethod as claimed in claim 18 comprising forming of said nubs as n-sidedpolygon.
 25. A method as claimed in claim 24 wherein n is between 3 and6.
 26. A method as claimed in claim 18 comprising rastering said uppersurface of said auxiliary layer with a raster structure comprising aplurality of n-sided polygons.
 27. A method as claimed in claim 26wherein n is between 3 and
 6. 28. A method as claimed in claim 18comprising forming of each of said nubs as an n-sided polygon, andrastering said upper surface of said auxiliary layer with a rasterstructure comprising a plurality of n-sided polygons.
 29. A method asclaimed in claim 28 wherein n is between 3 and 6.